ACTH stimulates adrenal steroidogenesis by increasing the rate of formation of pregnenolone in the mitochondria. Protein synthesis has long been implicated in this process. Present evidence indicates that ACTH does not stimulate protein synthesis but rather mediates the transformation of a rapidly turning-over protein to a form which is active in the stimulation of pregnenolone synthesis. This active form is most likely a phosphoprotein. We are concerned with detecting this protein, establishing its role in pregnenolone synthesis, and isolating it. To these ends 32Pi, 32P-ATP, and 14C-amino acids will be used under varying experimental conditions and gel-electrophoresis, particularly of the mitochondrial proteins, used as an initial method of detection.